This invention relates to a technique which is effective when applied to a moving picture decoding apparatus, and more particularly to a moving picture decoding apparatus suitable for displaying moving pictures and still pictures of high resolution.
In the conventional moving picture decoding apparatus, there are provided some line buffers each capable of storing data for one line of a picture, and those line buffers are used in turns to write and read data to thereby improve the data transfer efficiency. To give an example, when pixel data of a standard size of 352.times.240 dots as a format of a moving picture is outputted to form a picture of 704.times.480 dots which comply with the ordinary resolution of TV, the line buffers are changed over in the sequence as shown in FIG. 1, for instance.
In this example, three line buffers are used, one of them is allocated to writing of pixel data of the next line therein, and the remaining two line buffers are allocated to reading of pixel data therefrom. The reason why two buffers are required for reading is to realize data interpolation in the vertical direction to expand picture data for 240 lines to picture data for 480 lines. For vertical inter-line interpolation, it is necessary to simultaneously provide data for a line which is being displayed at present (hereafter referred to as main display data) and data for a line which is going to be displayed next (hereafter referred to as subsidiary display data). Based on those two items of data, interpolation data is calculated and displayed.
Assuming that one cycle consists of a sequence of writing and reading data for one line (hereafter referred to as 1H), an example of a manner of the change-over between writing and reading operation modes of the buffers is as shown in FIG. 1. While a line buffer for data writing is selected for every 1H cycle in the order of A to B to C, subsidiary display data are read from another selected buffer in the order of C to A to B, and main display data are read from yet another selected buffer in the order of B to C to A. If attention is paid to a certain buffer, in a 1H cycle after a particular 1H cycle where a data was written in that buffer, the written data is read from therefrom as subsidiary display data, and in the next 1H cycle, the written data is read again from that buffer as main display data. In the subsequent cycle, new data is written again in that buffer, and reading and writing are repeated in the same sequence.
The three buffers are controlled such that they perform a data writing operation, a subsidiary display data reading operation and main display data reading operation, respectively, and that their operations are changed-over at every 1H cycle among the three modes of operations cyclically. In displaying moving pictures, as shown in FIG. 2, since moving picture data contains 352 dots in comparison with a display size of 704 dots a line, each dot is displayed twice for each line.
For outputting 704.times.480 pixels in a typical high definition picture format on a screen of a 704.times.480 dot-size, vertical interpolation is not required, so that the number of line buffers required is two, one for writing and one for reading, and each line buffer needs to have a storage capacity of data for 704 pixels. In other words, in order to realize an apparatus capable of outputting both a standard picture (352.times.240 dots) and a high definition picture (704.times.480 dots) by the conventional method, it is necessary to prepare a line buffer for a standard picture, which is capable of storing data for 352 pixels, and two line buffers for a high definition picture, each of which is capable of storing data for 704 pixels.
In the conventional moving picture decoding apparatus, each line buffer is required to store all pixel data for one line. Therefore, when the amount of data for one line differs, for example, between a standard picture and a high definition picture as described above, there occurs a waste of line buffer capacity, and when a plurality of line buffers are prepared for a change-over between the modes of operation, reading and writing, the waste of the buffer capacity increases, resulting an increase of the cost of the apparatus.